1. Field of the Invention
This invention relates to an adhesive composition comprising polyolefins containing carbonyl groups and calcium chloride and/or magnesium chloride which provides improved waterproof and chemical-resistant adhesion between a metal and a polyolefin.
2. Description of the Prior Art
It has been a wide practice, as one method of preventing the corrosion of metals, to coat the surface of the metals with resins. Polyolefin resins such as polyethylene or polypropylene and polyvinyl chloride, for example, have been used for this purpose. From the standpoint of resistance to brittleness at low temperatures and food sanitation, a coating of polyethylene has especially been desired in many types of industries.
However, since polyethylene is non-polar, it is difficult to adhere polyethylene directly to a metal. Therefore, the adhesion of polyethylene to a metal has been increased by subjecting the polyethylene to a corona discharge treatment or by using an adhesive. As adhesives for adhering polyolefins to metals, ethylene copolymers containing polar groups (e.g., ethylene/vinyl acetate copolymers and polyethylene containing chlorosulfonic acid groups) are widely used.
These adhesives are superior in adhering polyolefins to metals, but almost all of them have defects in waterproof adhesiveness. When a metal is completely coated with a resin, the waterproofness of the adhesive scarcely poses a problem because the adhesive does not come into contact with water unless the resin coating breaks. However, in actual use of the coated metal, frequently the coated resin breaks and the base metal is exposed because of pinholes formed during the production of the resin, due to cutting, welding and thread-cutting operations conducted during the processing of the coated metals, or as a result of impact after processing. When the waterproof adhesiveness of the adhesive is poor, the coated resin peels off within short periods of time, and the corrosion protection of the metal is destroyed.
Chlorosulfonated polyethylene is now in use as a feasible waterproof adhesive for adhering polyolefins to metals. However, this adhesive has the defect that because of its poor heat stability, the temperature range at which it can be utilized is very narrow, and the handling of the adhesive is difficult. Further, since the synthesis of this adhesive requires complicated procedures, it is higher in cost than other adhesives of a similar nature. Some polyolefins containing polar groups exhibit equal or superior salt water-resistant adhesiveness to chlorosulfonated polyethylene, but a polyolefin containing polar groups which excels chlorosulfonated polyethylene both in salt water-resistant adhesiveness and salt crock-resistant adhesiveness has never been obtained.
In an attempt to improve the salt crock-resistant and salt water-resistant adhesive properties of ethylene copolymers containing a polar group, organic and inorganic additives have been extensively studied, and consequently it has now been found that only calcium chloride and magnesium chloride unexpectedly provide striking effects. Accordingly, this invention provides an adhesive composition comprising (1) a polyolefin having therein at least one repeating unit containing a carbonyl group and (2) at least one of calcium chloride, magnesium chloride or a mixture thereof.